The refractive index of a material depends on the frequency or wavelength of the light. Hence it is necessary to include both the value (a unitless number) and the frequency or wavelength. The frequency will be a quantity type with a frequency unit, such as Hz, kHz, or MHz. The wavelength will be a quantity type with a length unit, such as m, cm, um, in, or ft. The value of the refractive index. The frequency (and uom) for which the refractive index is measured. The wavelength (and uom) for which the refractive index is measured. The reported wavelength should be the wavelength of the light in a vacuum. Unique identifier for the index.